The present invention generally relates to cylindrical alkaline zinc/manganese dioxide electrochemical cell batteries.
Cylindrical alkaline zinc/manganese dioxide batteries are popular sources of power for portable devices used by consumers. They are readily available, highly reliable and provide good shelf life and discharge characteristics at a reasonable cost.
Portable devices are increasingly requiring higher rate output (current and power) capability, as well as increasingly longer discharge times from the batteries that power the devices. At the same time, the trend is toward more compact devices, hence smaller batteries.
One way to achieve higher rate output as well increase discharge times is to improve battery discharge efficiency. Batteries are able to deliver only a fraction of their theoretical capacity, and, in general, that fraction (the discharge efficiency) decreases as the discharge rate increases. One factor that can affect the discharge efficiency of a battery is the interfacial surface area between the electrodes in the battery's cell(s). Increasing the interfacial surface area generally has positive effects on current density, internal resistance, concentration polarization, and other characteristics that can effect discharge efficiency. Accordingly, the current, power and discharge capacity of alkaline batteries can be increased by increasing the interfacial surface area between the anodes and cathodes.
Typical consumer cylindrical alkaline Zn/MnO2 battery cells have a bobbin-type construction, with coaxially disposed electrodes. The positive electrode (cathode) has essentially a hollow cylindrical shape with a smooth, round internal surface disposed next to the cell container's sides. A negative electrode (anode) is disposed within the hollow cavity in the cathode, with a separator between the opposing anode and cathode surfaces (i.e., in the electrode interface). The area of that interface is the interfacial surface area, which can be approximated by measuring the area of the inner surface of the hollow cathode cylinder.
There have been previous attempts to improve the high power capability and/or the high rate discharge capacity of alkaline batteries by increasing electrode interfacial area. Examples can be found in U.S. Pat. No. 3,335,031, U.S. Pat. No. 5,869,205, U.S. Pat. No. 6,074,781 and U.S. Pat. No. 6,342,317. However, each of these references suffers from one or more of the following disadvantages, particularly for small diameter, small volume cells such as AAA/LR03 and AAAA/LR8D425 sizes.
Manufacture of cells is difficult when a current collector prong must extend into each of a plurality of like-polarity electrodes. This means that each current collector prong must be aligned with one of the plurality of electrodes, requiring orientation of both the cell and the current collector. In addition, when multiple current collector prongs are required, the volume of active materials must be reduced to allow for an increase in the total volume of the collector, compared to cell designs in which a single current collector prong will suffice.
When the shape of the electrode interface is not essentially a right cylinder (e.g., when there are radial projections), the separator can be difficult to insert because of irregular surfaces and small clearances. Typical separator materials (e.g., polymeric film and woven or non-woven paper or fabric) in strip or sheet form may not conform well to the surface of the cavity in the cathode. Even application of a spray-on separator to the interfacial surface of one of the electrodes can be difficult. Sharp corners and narrow recesses in the interfacial surface of the cathode can make it difficult to completely fill the cavities with anode material, especially at high manufacturing speeds. Active materials can be non-uniformly, therefore incompletely, consumed during discharge because the maximum distance from the electrode interface (and the opposite electrode) can vary considerably in different parts of both electrodes. Electrode lobes can be more fragile, resulting in damage during assembly and short circuits within the cells.
The relative increase in separator volume can be greater than the increase in anode and cathode volumes, at least partially offsetting increases in discharge capacity that could be achieved through improved discharge efficiency with decreases due to relative reductions in the amounts of active materials.
Another way to increase the discharge capacity of a battery is to increase the amount of active materials that are put into the cell. This can be difficult if the external dimensions of the battery are not increased, as may be the case when maximum dimensions are specified, as is often the case for standard battery types in various industry standards. Examples of this approach are found in U.S. Pat. Nos. 5,283,139 and 6,265,101. Each of these references suffers from the disadvantage that the maximum dimensions are maintained, limiting the amount of possible increase in the amount of active materials (and therefore the theoretical capacity) in the batteries.
Since it is often desirable to minimize the size of portable devices, it is likewise desirable to minimize the volume of the batteries that power those devices.
For the foregoing reasons there is a need for a cylindrical alkaline cell with increased theoretical capacity as well as increased discharge efficiency, while minimizing the increase in cell volume.